An identification medium that can be used for determining authenticity is disclosed in Japanese Unexamined Patent Application Laid-open No. 2009-172798, for example. In this identification medium, different images are viewed by observation through a right-handed or a left-handed circularly polarizing filter, whereby identification is performed. In this technique, the identification medium has a basic structure in which a light-transmissive layer, a printed layer forming a first pattern, and a cholesteric liquid crystal layer, are laminated in turn. The cholesteric liquid crystal layer is formed with a holographic pattern (second pattern) by embossing.
In the identification medium having the above basic structure, it is assumed that the cholesteric liquid crystal layer has a characteristic of selectively reflecting right-handed circularly polarized light. The identification medium may be observed through a circularly polarizing filter (left-handed circularly polarizing filter). The circularly polarizing filter selectively transmits left-handed circularly polarized light and is arranged apart from the identification medium. In this case, right-handed circularly polarized light is reflected at the cholesteric liquid crystal layer, but is cut off by the left-handed circularly polarizing filter and is thereby not perceived. That is, the second pattern formed on the cholesteric liquid crystal layer cannot be viewed.
On the other hand, light reflected at the printed layer includes random linearly polarized components. Therefore, light reflected at the first pattern passes through the left-handed circularly polarizing filter and is perceived. That is, in this case, only the first pattern of the printed layer is selectively viewed.
In contrast, the identification medium may be observed through a circularly polarizing filter (right-handed circularly polarizing filter) that selectively transmits right-handed circularly polarized light. In this case, right-handed circularly polarized light reflected at the cholesteric liquid crystal layer passes through the right-handed circularly polarizing filter, whereby the second pattern is visible. In addition, the first pattern is faintly visible at the same time.
The first pattern is visible for the following reason. In this case, components other than right-handed circularly polarized light having a predetermined wavelength pass through the cholesteric liquid crystal layer and include linearly polarized components in every direction. The linearly polarized components are reflected at the printed layer and pass through the cholesteric liquid crystal layer, and then enter the right-handed circularly polarizing filter. Since the right-handed circularly polarizing filter transmits some of the linearly polarized components, the printed layer is faintly visible.
Thus, in the above technique, the first pattern is visible in each of the observation using the right-handed circularly polarizing filter and the observation using the left-handed circularly polarizing filter. However, in view of optical identifying function, the patterns are preferably clearly altered by switching the right-handed and the left-handed circularly polarizing filter.